Method for producing strained quantum well semiconductor laser elements

ABSTRACT

A semiconductor laser element having a GaAs substrate formed thereon with an active layer of a strained quantum well construction provided with an In x  Ga 1-x  As strained quantum well layer and a GaAs barrier layer, and clad layers arranged up and down said active layer through an epitaxial growth means. The lattice mismatching rate of the clad layer with respect to the substrate is less than 10 -3 .

This application is a division of application Ser. No. 07/606,812, filedOct. 31, 1990.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor laser elements of astrained quantum well type and a method for their production.

2. Description of Prior Art

A semiconductor laser element formed on a GaAs substrate with an activelayer of a strained quantum well construction provided with an In_(x)Ga_(1-x) As (x=0.0 to 0.05) strained quantum well layer and a GaAsbarrier layer is already known as a light source of wavelength of 0.9 to1.1 μm. This is a conventional lattice matched type laser, such asGaAs/AlGaAs and InAsP/InP.

In the case of semiconductor laser elements, as a clad for confiningcarrier and light in an active layer, a semiconductor should be usedwhich has a permeability with respect to light having an oscillationwaveform, is smaller in refractive index than that of the active layer(or a layer for confining light near the active layer), and is large inits energy gap.

In the conventional semiconductor laser element of an In_(x) Ga_(1-x) Asstrained quantum well type, Al_(w) Ga_(1-w) As of w>0.2 is used as aclad.

FIG. 8 shows a conventional semiconductor laser element of an In_(x)Ga_(1-x) As strained quantum well type.

In FIG. 8, an n-type GaAs substrate 1 having approximately 350 μm ofthickness is formed thereon with an n-type GaAs buffer layer 2 havingapproximately 0.5 μm of thickness and an n-type Al₀.3 Ga₀.7 As cladlayer 3 having approximately 1.5 μm thickness through epitaxial growthmeans such as the MBE or MOCVD method.

Further, in FIG. 8, the n-type GaAs substrate 1 is formed at itspredetermined position with an essential portion 4 including an activelayer provided with an In₀.35 Ga₀.65 As strained quantum well layer, aGaAs barrier layer, etc., and a light confining layer, a p-type Al₀.3Ga₀.7 As clad layer 5 having 1.5 μm of thickness and a p-type contactlayer 6 having 0.2 μm of thickness.

The details of part 4 are clearly shown in FIG. 9.

In FIG. 9, two upper and lower GaAs light confining layers 7 are 1500 Åin thickness, a GaAs barrier layer 8 between these light confininglayers 7 is 100 Å in thickness and an In₀.35 Ga₀.65 strained quantumwell layer 9 is 40 Å in thickness.

A double hetero construction uniformly formed on the GaAs substrate 1has a current restricting layer and an electrode, applied withmicroworking such as element separation, to prepare a laser chip.

One example of prior art has been described above. The semiconductorlaser element has various modes such as a ratio of composition of mixedcrystal, the number of layers of strained quantum wells, thickness ofthe layers, etc. As the light confining layer, the well known GRINSHCconstruction is used, including AlGaAs in which the ratio of the Alcomposition is parabolic.

Next, the process for working the bridge waveguide path into a strainedquantum well type semiconductor laser element will be describedhereinafter with reference to FIGS. 10(a) to 10(d).

In the process shown in FIG. 10(a), a resist 10 is patterned on a p-typeGaAs contact layer 6 through means such as photolithgraphy.

In the process shown in FIG. 10(b), the resist 10 is used as a mask. TheGaAs contact layer 6 of the double hetero construction and the upperclad layer 5 are subjected to etching until the depth of the etchingreaches about 0.2 m of the active layer 4.

As etching liquids of the AlGaAs/GaAs type, there can be used a mixedsolution of sulfuric acid and hydrogen peroxide, a mixed solution oftartaric acid and hydrogen peroxide, a mixed solution of ammonia andhydrogen peroxide or dry etching such as chlorine (for example, reactiveion beam etching).

In the process of FIG. 10(c), means such as spattering is used to form asurface of an epitaxial film with an etching mask 11 in the form of afilm such as SiO₂, SiN, etc.

In the process step of FIG. 10(d), means such as photolithgraphy is usedto form an etching portion 12 in a stripe-like SiO₂ as a patternedresist mask.

In the process steps that follow, electrodes are formed on both upperand lower surfaces of a laminate construction, and microworking, such aselement separation, is applied thereto.

The technical tasks with regard to the aforementioned semiconductorlaser element include oxidation of the Al, and providing compressivestress from a substrate lattice and defective etching process, whichwill be described hereinafter.

The main uses of laser having a 0.9-1.1 μm waveform are excitation of afiber amplifier in which a rare earth, such as Er is doped, or a visuallight source in combination with SHG. In the cases of thesesemiconductor products, a prolonged service life is required at highoutput in excess of scores of mW.

However, the conventional strained quantum well type semiconductor lasercannot fulfill such a requirement as described above, since it uses theaforementioned AlGaAs as a clad layer. The reason is as follows.

Among elements (for example, In, Al, Ga, As, P, Sb, etc.) constituting acompound semiconductor, Al is an element which tends to be oxidized mosteasily. For example, when a regrowth surface is made by an embeddinggrowth means during fabrication of a laser chip, oxidation of Al tendsto occur.

Such an Al oxidation results in the occurrence of a non-light emittingcenter and the degradation of crystallization, and failing to obtain asemiconductor laser excellence in the laser characteristics.

Furthermore, in the case where a plaited or weave-link surface is usedas a laser end, oxidation of the end progresses during use of laser tobring forth a lowering of refractive index, and an increase ofabsorption, deteriorating laser characteristic.

The semiconductor laser increases its temperature particularly duringlaser operation at high pouring, the progress of oxidation being spedup.

Means are proposed to remove oxygen and water content in the fabricationprocess of the laser, in order to suppress oxidation of Al. However,this requires much labor.

Because of this, in case of prior art, it is not possible to obtain alaser having a long service life under the use condition of high output.

In a double hetero construction for a laser diode using a conventionalIN_(x) Ga_(1-x) As/GaAs strained quantum well construction as an activelayer, semiconductors having a larger lattice constant than that of GaAssubstrate are laminated.

That is, the lattice constant of the GaAs substrate is 5.65 Å whereasAl_(w) Ga_(1-w) As has a large lattice constant, 0.14 w % and In_(x)Ga_(1-x) As has a large lattice constant, 7.3 x %.

Incidentally, assume that compositions of In_(x) Ga_(1-x) As and Al_(w)Ga_(1-w) As are x=0.35 and w=0.3, respectively, lattice non-matchingrates with respect to GaAs are+2.65% and+0.04%, respectively.

In this case, the Al_(w) Ga_(1-w) As layer is small in the latticenon-matching rate with respect to GaAs but is thick, 3 μm (about tenthousand atom layer). An In_(x) Ga_(1-x) As layer is thin, 120 Å (about40 atom layer), but the lattice non-matching rate with respect to GaAsis large and, therefore, a laminate of the Al_(w) Ga_(1-w) As layer andthe In_(x) Ga_(1-x) As layer receives a compressive stress from thesubstrate. This compressive stress causes occurrence of transition andslip in the active layer of the strained quantum well constructionduring high pouring and the laser operation of high excitation.

As a result, the semiconductor laser element tends to give rise to DLD(dark line defect), lowering the laser oscillation life.

Defective Etching Process

In order to control a lateral mode, the semiconductor laser element isused with a guide wave mechanism of either the gain guide wave type orthe refractive index guide wave type.

With respect to these guide wave types, various laser elementconstructions have been proposed. In case of the strained quantum welltype semiconductor laser element, a lattice mismatching is presentbetween a well and a barrier, and, therefore, an active layer issubjected to mesa etching to have a stripe configuration, after which alayer is embedded therein and grown.

However, in the laser element such as a BH construction, a defect suchas transition tends to occur in the embedded growth layer near theactive layer, making it difficult to obtain a semiconductor laserelement having a long life.

On the other hand, in a ridge waveguide laser element, the element canbe produced leaving an active layer to be flat, which is, therefore, oneof laser elements suitable for the strained quantum well type.

This ridge waveguide laser element can be produced by means illustratedin FIG. 10.

Among the steps shown in FIGS. 10(a) to 10(b), the mesa forming processin FIG. 10(b) is important, in order to control the lateral mode, toaccurately control the distance between the mesa bottom and the activelayer and in order not to scatter light, to finish the mesa bottom flat.

However, severe control is required with respect to the concentrationand temperature of the etching liquid in accurately controlling thedepth of etching and, similarly, the concentration and temperature ofthe etching liquid should be maintained uniformly so that the depth ofetching is not uneven within the etching surface even when etching iscarried out. Therefore, the technical difficulty of this importantprocess increases.

As a result, it is difficult to produce a semiconductor laser elementhaving excellent laser characteristics with good reproduceability,lowering the yield of good products.

In view of the aforesaid technical task, the present invention providesa semiconductor laser element which exhibits an excellent lasercharacteristic for a long period of time, and a method for theproduction of the semiconductor laser element.

SUMMARY OF THE INVENTION

According to a first feature of the present invention, there is provideda semiconductor laser element comprising an active layer of a strainedquantum well construction provided with an In_(x) Ga_(1-x) As strainedquantum well layer, a GaAs barrier layer and clad layers arranged up anddown of said active layer. The active layer and clad layers are formedon a GaAs substrate through an epitaxial growth means, the clad layerbeing formed of In_(z) Ga_(1-z) As_(y) P_(1-y).

In this case, it is desired that the lattice mismatching rate of theclad layer to the substrate be less than 10⁻³ as described in claim 2.

According to a further feature (claim 3), there is provided asemiconductor (claim 1) wherein a stress relieving layer comprisingIn_(1-z) Ga_(z) P (z 0.51) is interposed between the active layer andthe upper and lower clad layers.

According to another feature (claim 4), there is provided asemiconductor laser element comprising an active layer of a strainedquantum well construction provided with an In_(x) Ga_(1-x) As strainedquantum well layer and a GaAs barrier layer, and clad layers arranged upand down the active layer, the active layer and the clad layers beingform on a GaAs substrate through an epitaxial growth means, the cladlayer being formed of InGaP and having a GaAs etching stop layerinserted therein.

Also in this case, it is desired that the lattice mismatching rate ofthe clad layer to the substrate be less than 10⁻³ as described in claim4.

According to another feature (claim 6) of the present invention, thereis provided a method for the production of a semiconductor laser elementcomprising an active layer of a strained quantum well constructionprovided with an In_(x) Ga_(1-x) As strained quantum well layer and aGaAs barrier layer, InGaP clad layers arranged up and down of saidactive layer and a GaAs etching stop layer inserted into said cladlayer, said active layer, said clad layers and said etching stop layerbeing formed on a GaAs substrate through an epitaxial growth means, themethod comprising the steps of etching an InGaP layer to a GaAs layerwith an etching liquid containing either sulfuric acid, tartaric acid orammonia and hydrogen peroxide, and etching an InGaP layer to a GaAslayer with an etching liquid containing hydrochloric acid but notcontaining hydrogen peroxide.

The functions of the present invention will be described. (1)Semiconductor laser element of Claim 1:

In the semiconductor laser element, in order that clad layers areprovided up and down the active layer may sufficiently confine light andcarriers in the strained quantum well active layer and a light confininglayer, it is necessary that the former is lattice matched with the GaAssubstrate, the refractive index with respect to light having 0.9 to 1.1μm of wavelength is Al_(w) Ga_(1-w) As (w=0.5-0.6) and the energy gap islarge equal to Al_(w) Ga_(1-w) As (w=0.5-0.6).

In the semiconductor laser element according to Claim 1, each of upperand lower clad layers of the active layer is formed of In_(z) Ga_(1-z)As_(y) P_(1-y).

The In_(z) Ga_(1-z) As_(y) P_(1-y) is fulfilled with the aforesaidcondition as the clad layer by adjusting the composition, and does notcontain Al, and therefore, there creates no problem, as mentioned above,caused by Al oxidation.

Of course, in a strained quantum well type semiconductor laser elementaccording to Claim 1 not only the clad layer but all of the structuralmembers are free of Al, which is desirable.

(2) In the semiconductor laser element according to Claim 1, if themismatching rate of the clad layer with respect to the substrate is lessthan 10⁻³, as in Claim 2, both members become substantially completelylattice-matched.

(3) In the semiconductor laser element according to Claim 3:

The lattice constant a of In_(1-z) Ga_(z) P is given by the followingformula (1) according to the Vegad rule. When z=0.51, lattice matchingwith GaAs is obtained, and when z>0.51, the lattice constant is smallerthan that of GaAs.

    a=5.869-0.42×z                                       (1)

In case of the semiconductor laser element according to Claim 3, anIn_(1-z) Ga_(z) P stress relieving layer having a "z" which is smallerin lattice constant than that of GaAs substrate is provided near thestrained quantum well active layer.

It is desired that the In_(1-z) Ga_(z) P stress relieving layer isdesigned so that an average lattice constant a of an epitaxial layer,given by the following formula (2), is equal to the lattice constant ofthe GaAs substrate.

    a=Σa.sub.i t.sub.i /Σ.sub.i                    ( 2)

a_(i) : lattice constant of each epitaxial layer

t_(i) : thickness of each epitaxial layer

Since the thus designed epitaxial layer is lattice-matched to thesubstrate, stress in an interface between the substrate and theepitaxial layer hardly occurs, and occurrence of transition at theinterface can be suppressed.

Accordingly, the strained quantum well type semiconductor laser elementaccording to Claim 1, has a long service life.

(3) Semiconductor laser element of Claim 4:

The semiconductor laser element according to Claim 4 is configured bymaking use of properties of InGaP that it is lattice-matched to GaAs,and InGaP and GaAs can be subjected to selective etching at differentspeeds.

That is, in a double hetero construction for laser having an activelayer of a strained quantum well construction with a ridge waveguidetype, a clad layer is formed of InGaP, and A GaAs etching stop layer isinserted at a control position of an etching depth in a mesa formingstep into the InGaP clad layer. Therefore when such a double heteroconstruction is subjected to selective etching processing, it ispossible to accurately control the distance between the mesa bottom andthe active layer.

(5) Also in the case of the semiconductor laser element according toClaim 4, if the lattice mismatching rate of the clad layer to thesubstrate plate is less than 10⁻³ as in Claim 5, both the members becomesubstantially completely lattice-matched.

(6) Method for the production of a semiconductor laser element accordingto Claim 6:

The method according to Claim 6 is the method for producing thesemiconductor laser element according to Claim 4.

In the mesa forming step according to Claim 4, either a mixed liquid ofsulfuric acid and hydrogen peroxide, a mixed liquid of tartaric acid andhydrogen peroxide or a mixed liquid of ammonia and hydrogen peroxide isused to apply etching to a GaAs contact layer, and an etching liquidcontaining hydrochloric acid but not containing hydrogen peroxide isused to apply etching to an InGaP clad layer.

In case of one of said etching liquids containing hydrogen peroxide, theetching speed varies depending on concentration and temperature.However, the etching speed with respect to GaAs is normally 20 times ormore of the etching speed with respect to InGaP.

In case of the other etching liquid not containing hydrogen peroxide,the etching speed with respect to GaAs is normally one ten-thousandth ofthe etching speed with respect to InGaP.

Accordingly, in the mesa forming step in the method according to Claim6, first, the aforementioned one etching liquid containing hydrogenperoxide can be used to selectively etch the GaAs layer alone.Subsequently, the other etching liquid can be used to selectively etchthe InGaP clad layer up to the GaAs etching stop layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a double hetero type layerconstruction as a first embodiment of a semiconductor laser elementaccording to the present invention;

FIG. 2 is an enlarged view of essential parts of the semiconductor lasershown in FIG. 1;

FIG. 3 is a sectional view showing an embedded type layer constructionas a second embodiment of a semiconductor laser element according to thepresent invention;

FIG. 4 is a sectional view showing a double hetero type layerconstruction as a third embodiment of a semiconductor laser elementaccording to the present invention;

FIG. 5 is a sectional view showing a ridge waveguide layer constructionas a fourth embodiment of a semiconductor laser element according to thepresent invention;

FIG. 6 is a sectional view showing a double hetero type layerconstruction as a fifth embodiment of a semiconductor laser elementaccording to the present invention;

FIG. 7 is a sectional view showing an SBA type layer construction as asixth embodiment of a semiconductor laser element according to thepresent invention;

FIG. 8 is a sectional view showing a double hetero type layerconstruction as a conventional semiconductor laser element;

FIG. 9 is an enlarged view of the essential parts of the semiconductorlaser element shown in FIG. 8; and

FIG. 10(a) to 10(b) are explanatory views showing the steps of preparinga conventional double hetero construction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A method for the production of a semiconductor laser element accordingto the present invention will be described in connection with theembodiments shown.

FIGS. 1 and 2 show a first embodiment of a semiconductor laser elementaccording to the present invention. A semiconductor laser element of thefirst embodiment has a structure which will be described below.

In FIG. 1, an n-type GaAs substrate 1 having about 350 μm of thicknessis formed thereon with a n-type GaAs buffer layer 2 having about 0.5 μmof thickness and an n-type In₀.49 Ga₀.51 As clad layer 3a having about1.5 μm of thickness through epitaxial growth means such as MBE methodand MOCVD method and further at a predetermined position thereof with amain portion 4 including an active layer provided with an In₀.35 Ga₀.65As strained quantum well layer, a GaAs barrier layer and the like and alight confining layer, a p-type In₀.49 Ga₀.51 As clad layer 5a havingabout 1.5 μm of thickness and a p-type contact layer 6 having about 0.2μm of thickness.

With the structure of the main portion 4 clearly shown in FIG. 2, twoupper and lower GaAs light confining layers 7 have 1500 Å of thickness,a GaAs barrier layer 8 between the light confining layers 7 has 100 Å ofthickness, and an In₀.35 Ga₀.65 As distortion quantum well layer 9 has40 Å of thickness.

The double hetero construction uniformly formed on the GaAs substrate 1is formed with a current restricting layer and an electrode, as isknown, and is applied with microworking such as element separation toprepare a laser chip.

The semiconductor laser element of the present invention illustrated inFIGS. 1 and 2 is different from that shown in FIGS. 8 and 10, in thatone clad layer 3a is formed of p-type In₀.49 Ga₀.51 As and the otherclad layer 5a is formed of n-type In₀.49 Ga₀.51 As.

The front electrode type semiconductor laser element having theconstruction as described above has 140 A/cm² of oscillation thresholdcurrent density during the pulse drive under room temperature.

This value is equal to that in which both clad layers are formed ofAl₀.50 Ga₀.5 As.

FIG. 3 shows a second embodiment of a semiconductor laser elementaccording to the present invention, and an embedded type semiconductorlaser element of the second embodiment has the structure which will bedescribed below.

In FIG. 3, an n-type GaAs substrate 11 is formed thereon with a mainportion 4 including an n-type GaAs buffer layer 12, an n-type InGaP cladlayer 13, an active layer of a strained quantum well construction and alight confining layer, a p/n junction p-type InGaP blocking layer 15 andan n-type InGaP blocking layer 16, a p-type InGaP clad layer 17 and ap-type GaAs cap layer 18 through the aforementioned epitaxial growthmeans and etching means and further on the back side thereof with ap-type electrode 19 of AuZn type, the p-type GaAs cap layer 18 beingformed on the upper surface thereof with an n-type electrode 20 ofAuGeNi type.

In FIG. 3, the main portion 14 including an active layer of distortionquantum well construction and a light confining layer has 1.5 μm ofwidth and 600 μm of length of resonance unit, and a cleavage surfacethereof is in the form of a mirror without a protective film.

When the semiconductor laser element of the second embodiment shown inFIG. 3 is operated under room temperature, CW oscillation is obtained atpouring of 7 mA, and output of 100 mW is obtained at a pouring of 600mA. After laser drive 50 mW and 200 hours, the I-L characteristicremains unchanged. A long service life was maintained under the highoutput state.

While in the second embodiment, InGaP has been used as a clad layer, itis to be noted that even if InGaAsP having 1.5 eV or more of energy gapis used as a clad layer, light and carrier can be confined.

Since InGaP as well as InGaAsP contain no Al, oxidation of regrowthinterface and cleavage surface is hard to occur and accordingly a laserof long service life can be prepared.

With respect to InGaAsP, lattice matching with the GaAs substrate may beprovided to a degree that transition does not occur.

In the second embodiment, the film thickness of layers, composition, andthe number of strained quantum well layers are not limited to theaforementioned contents and the illustrated examples.

In the second embodiment, the GRIN construction in which the compositionof InGaAsP is stepwisely changed is sometimes employed as a lightconfining layer.

In case of the semiconductor laser element of the second embodimentillustrated in FIG. 3, the cleavage surface is sometimes subjected tocoating for the purpose of improving low threshold, efficiency, andoutput.

FIG. 4 shows a third embodiment of a semiconductor laser elementaccording to the present invention. The semiconductor laser element ofthe third embodiment has the structure described below.

In FIG. 4, an n-type GaAs substrate 31 having 350 μm of thickness formedthereon with an n-type GaAs buffer layer 32 having 0.5 μm of thickness,an n-type In₀.49 Ga₀.51 As clad layer 33 having 1.5 μm of thickness, ap-type In₀.47 Ga₀.35 P stress relieving layer having 0.12 μm ofthickness, a main portion 40 including an active layer and a lightconfining layer, an n-type In₀.47 Ga₀.53 stress relieving layer 42having 0.12 μm of thickness, a p-type In₀.49 Ga₀.51 As clad layer 35having 0.5 μm of thickness, and a p-type contact layer 36 having 0.2 μmof thickness in order through the aforementioned epitaxial growth means.

The structure of the main portion 40 in FIG. 4 is the same as that formentioned in FIG. 2. Parts constituting the main portion 40 areindicated by reference numerals in FIG. 2.

That is, in FIG. 2, also showing the main portion 40 of FIG. 4, two GaAslight confining layers 37 positioned up and down have 1500 Å ofthickness, each GaAs barrier layer 38 between these light confininglayers 7 has 100 Å of thickness, and each In₀.35 Ga₀.65 As strainedquantum well layer 39 has 40 Å of thickness.

An average lattice constant between an In₀.35 Ga₀.65 As strained quantumwell layer 39 having 40 Å thickness and+2.5% of lattice mismatching rateand an In₀.47 Ga₀.53 P stress relieving layers 41 and 42 having 1200 Åthickness and-0.13% of lattice mismatching rate is substantially equalto that of the GaAs substrate 31.

Let Δ a/ao be the lattice mismatching rate of the epitaxial growth layerwith respect to the substrate 31, this value is very small as shown bythe formula (3) below.

    Δa/ao=1.2×10.sup.-6                            (3

As a comparative example 1, as shown in FIGS. 8 and 9, on an n-type GaAssubstrate 1 having 350 μm of thickness are laminated in order of ann-type GaAs buffer layer 2 having 0.5 μm of thickness, an n-type Al₀.3Ga₀.7 As clad layer 3 having 1.5 μm of thickness, a main portion 4including an active layer and a light confining layer, a p-type Al₀.3Ga₀.7 As clad layer 5 having 1.5 μm of thickness, and a p-type contactlayer having 0.2 μm of thickness to prepare a predetermined specimen.

In this case, the GaAs light confining layer 7 of the main portion 4 has1500 Å of thickness, the GaAs barrier layer 8 between the lightconfining layers 7 has 100 Å of thickness, and the In₀.35 Ga₀.65 Asstrained quantum well layer 9 has 40 Å of thickness.

Each double hetero construction of the third embodiment and comparativeexample 3 was worked into a complete electrode type laser element having300 μm of cavity length, and were pulse oscillated under roomtemperature. The oscillation threshold current density was measured toobtain results given in Table 1 below.

                  TABLE 1                                                         ______________________________________                                                    Threshold current density (A/cm.sup.2)                            ______________________________________                                        Third Embodiment                                                                            280 ± 20                                                     Comparative   280 ± 20                                                     Example 1                                                                     ______________________________________                                    

FIG. 5 shows a fourth embodiment of a semiconductor laser elementaccording to the present invention.

The semiconductor laser element of the fourth embodiment shown in FIG. 5is provided with a SiO₂ insulating film 43 and alloy electrodes 44 and45 while in the third embodiment, the element is prepared in the form ofridge wave guide type strained quantum well type.

In FIG. 5, reference numeral 46 denotes a flow of current.

In Comparative Example 2 to be compared with the fourth embodiment, theelement of the Comparative Example 1 is formed into the strained quantumwell type of the ridge wave guide type similar to that of the fourthembodiment.

With respect to the elements of the fourth embodiment and ComparativeExample 2, the threshold current density and life were measured toobtain the result shown in Table 2 below.

                  TABLE 2                                                         ______________________________________                                                  Threshold current density                                                     (mA)            Life                                                ______________________________________                                        Fourth Embodiment                                                                         12                2000 hr or                                                                    more                                            Comparative 13                 500 hr                                         Example 2                                                                     ______________________________________                                    

The life in Table 2 was determined according to 10% (30° C.) rise ofdrive current by APC of 50 mW.

It is found from the above measured results that the threshold currentand life of the strained quantum well type semiconductor laser elementaccording to the present invention were remarkably improved.

In case of the third and fourth embodiments, the clad layer, Al_(w)Ga_(1-w) As of w>0.3 is sometimes used.

In the third and fourth embodiment, it is desired that the averagelattice constant of the epitaxial layer is equal to that of the GaAssubstrate. However, an allowable range of the mismatching rate isapproximately |Δ a/ao|<1.2×10⁻⁴.

The thickness of layers, composition and number of strained quantum welllayers are not limited thereto but the GRIN construction is sometimesemployed as a light confining layer.

FIG. 6 shows a fifth embodiment of a semiconductor laser elementaccording to the present invention. The semiconductor laser element ofthe fifth embodiment has the structure described below.

In FIG. 6, an n-type GaAs substrate 51 having 350 μm of thickness isformed thereon with an n-type GaAs buffer layer 52 having 0.5 μm ofthickness, an n-type In₀.49 Ga₀.5 As clad layer 53 having 1.5 μm ofthickness, a main portion 60 including an active layer and a lightconfining layer, a p-type In₀.51 Ga₀.49 P clad layer 55a having 0.2 μmof thickness and a p-type In₀.51 Ga₀.49 P clad layer 55b having 1.3 μmof thickness, a p-type GaAs etching stop layer 61 interposed betweenboth the p-type clad layers 55a and 55b, and a p-type contact layer 56having 0.2 μm of thickness through the aforementioned epitaxial growthmeans.

The structure of the main portion 60 in FIG. 6 is the same as thatmentioned in FIG. 2. Parts constituting the main portion 60 areindicated by reference numerals in FIG. 2.

That is, in FIG. 2 also showing the main portion 60 in FIG. 4, two GaAslight confining layers 57 positioned up and down have 1500 Å ofthickness, each GaAs barrier layer 58 between the light confining layers7 has 100 Å of thickness, and each In₀.35 Ga₀.65 As distortion quantumwell layer 59 has 40 Å.

It is desired that the thickness of the etching stop layer 61 be lessthan 0.2 μm so as not to impair a light confining effect caused by theclad layer.

As a modified form of the fifth embodiment, in the same construction asthat of the fifth embodiment, only the thickness of the etching stoplayer 61 is set to 50 Å, and an absorption end of the etching stop layer61 is sometimes shifted to a short wave side by the quantum effect.

In the method for production of a semiconductor laser element accordingto the present invention, the step of mesa forming a semiconductor laserelement of the fifth embodiment is executed as described hereinafter.

First, when a GaAs contact layer 56 is subjected to etching, a mixedliquid of sulfuric acid and hydrogen peroxide is used.

The etching solution of the mixed solution varies with the conditionssuch as temperature, mixing ratio, stirring state of the solution.

However, since the etching speed of the GaAs by the mixed solution is 20or more times of InGaP, such a mixed solution is used whereby only theGaAs can be etched.

Next, when an InGaP clad layer 57 is subjected to etching until reachingthe etching stop layer 51, hydrochloric acid of 36% (weight percentage)is used.

The etching speed of InGaP by hydrochloric acid at 20° C. isapproximately 0.1 μm/sec., and an InGaP layer having 1.3 μm of thicknesscan be etched in 13±2 seconds by using the hydrochloric acid.

On the other hand, the etching speed of GaAs by hydrochloric acid is sosmall that it cannot be measured, such as 0.1 A/sec.. For example, incase of GaAs layer of 50 Å, even if the etching time of 10 minutes haspassed, it cannot be etched.

In this manner, by carrying out etching for 15 seconds usinghydrochloric acid, the etched depth of the InGaP clad layer is less thana single atomic layer (3 Å).

Furthermore, if the aforementioned double hetero construction isprepared by a precise crystal growth method such as the MCVD method orMBE method, a very falt film is obtained.

Incidentally, in the case where a device size is about 300 μm square, aflatness of atomic layer level can be obtained, and an error in filmthickness is less than 1%.

Accordingly, in the mesa forming step of the present method, if such adouble hetero construction is used, the depth of mesa can be designedwith the range of an error less than 1%, and a ridge waveguide laserelement of a strained quantum well construction having a flatness inwhich an etching bottom is at level of an atomic layer can be easilyprepared.

Double hetero constructions of the aforementioned fifth embodiment, themodified form and the prior art are prepared three times by use of theMOCVD method, and 100 ridge waveguide laser elements having a strainedquantum well construction were prepared from wafers thereof.

With respect to the semiconductor laser elements of the fifthembodiment, the modified example and prior art, the oscillationthreshold currents in 0.93±0.101 μm of oscillation wavelength weremeasured to obtain the results shown in Tables 3-1, 3-2 and 3-3 below.

                  TABLE 3-1                                                       ______________________________________                                        (Fifth Embodiment)                                                                    1st time      2nd time      2nd time                                  ______________________________________                                        - I th    12.3   mA       12.4 mA     12.6 mA                                 σ Ith                                                                              0.40  mA        0.40                                                                              mA      0.35                                                                              mA                                 ______________________________________                                    

                  TABLE 3-2                                                       ______________________________________                                        (Modified Example of Fifth Embodiment)                                                1st time      2nd time      2nd time                                  ______________________________________                                        - I th    10.2   mA       9.7  mA     10.1 mA                                 σ Ith                                                                             0.35   mA       0.35 mA     0.40 mA                                 ______________________________________                                    

                  TABLE 3-3                                                       ______________________________________                                        (Prior Art)                                                                           1st time      2nd time      2nd time                                  ______________________________________                                        - I th    25.2   mA       18.0 mA     30.4 mA                                 σ Ith                                                                             3.2    mA       2.9  mA     4.1  mA                                 ______________________________________                                    

As will be apparent from the above tables, the double heteroconstruction according to the embodiments of the present invention issuperior to the prior art in the average value of threshold current.Particularly, in the modified example of the fifth embodiment, since theabsorption of the etching stop layer at 0.93 μm of oscillationwavelength is small, the threshold value is smaller than that of thefifth embodiment.

Moreover, the double hetero construction in the embodiment of thepresent invention shows favorable results as compared with prior artwith respect to irregularities of thresholds, between batches as well aschips.

While in the above-described embodiment, the ridge waveguide laser hasbeen employed, it is to be noted that as a sixth embodiment, an etchingstop layer can be provided also in the laser construction of SAB typeillustrated in FIG. 7.

In FIG. 7, an n-type GaAs substrate 31 is formed thereon with an n-typeInGaP clad layer 33, an etching stop layer 38, a p-type InGaP blockinglayer 37, an active layer 34, an n-type InGaP clad layer 35 and ann-type GaAs contact layer 36.

In the etching step for preparing a SAB type semiconductor laser elementof FIG. 7, as an etching liquid for selectively etching InGaP, a mixedsolution of hydrochloric acid and phosphoric acid or hydrochloric acidand acetic acid is sometimes used.

The semiconductor laser element according to the present invention hasthe following effects.

According to a first effect, in a semiconductor laser element having aGaAs substrate formed thereon with an active layer of a strained quantumwell construction provided with an In_(x) Ga₃₁ x As strained quantumwell layer and a GaAs barrier layer, and clad layers arrange up and downsaid active layer through epitaxial growth means, said clad layer isformed of In_(z) Ga_(1-z) As_(y) P_(1-y) and, therefore, a strainedquantum well type semiconductor laser element having a large output along service life can be obtained.

In this case, if the lattice mismatching rate of the clad layer withrespect to the substrate is less than 10⁻³, a more favorable effect isobtained.

According to a further effect, in the aforementioned double heteroconstruction, since a stress relieving layer formed of In_(1-z) Ga_(z) Pis provided adjacent to the active layer, a strained quantum well typesemiconductor laser element having a long service life can be obtainedin view of the foregoing.

According to another effect, in a semiconductor laser element having aGaAs substrate formed thereon with an active layer of a strained quantumwell construction provided with an In_(x) Ga_(1-x) As strained quantumwell layer and a GaAs barrier layer and clad layers arranged up and downof said active layer through an epitaxial growth means, said clad layeris formed of InGaP, and a GaAs etching stop layer is inserted into saidclad layer, and therefore, such a double hetero construction is appliedwith etching processing whereby the distance between a mesa bottom andthe active layer can be accurately controlled, and as a semiconductor,an irregularity of threshold current is reduced and a yield of productis enhanced.

Also in this case, if the lattice mismatching rate of the clad layerwith respect to the substrate is less than 10⁻³, more favorable effectis obtained.

Moreover, according to the method for the production of theaforementioned semiconductor laser element according to the presentinvention, the method comprises the steps of etching a GaAs layer to anInGaP layer with an etching liquid containing either sulfuric acid,tartaric acid or ammonia, together with hydrogen peroxide, and etchingan InGaP layer to a GaAs layer with an etching liquid containinghydrochloric acid but not hydrogen peroxide. Therefore, one etchingliquid containing hydrogen peroxide can first be used to selectivelyetch only the GaAs layer, and said other etching liquid not containinghydrogen peroxide is then used to selectively etch the InGaP clad layerup to the GaAs etching stop layer, whereby a semiconductor having anexcellent characteristics can be easily prepared.

What is claimed is:
 1. A method for the production of a semiconductorlaser element having a GaAs substrate formed thereon with an activelayer of a strained quantum well construction provided with an In_(x)Ga_(1-x) As strained quantum well layer and a GaAs barrier layer, InGaPclad layers arranged up and down said active layer and a GaAs etchingstep layer inserted into said clad layer through an epitaxial growthmeans, the method comprising the steps of: etching a GaAs layer to anInGaP layer with an etching liquid containing hydrogen peroxide and anyone of sulfuric acid, tartaric acid or ammonia, and etching an InGaPlayer to a GaAs layer with an etching liquid containing hydrochloricacid but not containing hydrogen peroxide.